Washing of filters containing granular filter material, which becomes necessary after a certain, variably long period of operation of the filter is conventionally carried out by reverse flow of washing fluids. A number of methods of washing are known, namely reversing the flow of water by itself or stirring the filter mass with air by itself and flushing with water without air, or combined washing with air and water which are injected simultaneously into the mass. This latter type of washing is particularly advantageous because it uses greater stirring energies and requires shorter washing periods. It has been employed for a long time for washing sand filters. Today its efficiency no longer needs to be demonstrated. However, until now, combined washing with air and water (or hydropneumatic washing) could not easily be applied to the washing of filters containing lightweight materials or multilayer filters. In fact, lightweight materials, such as anthracite, have a low density of the order of 1500 kg/m.sup.3. A phenomenon of flotation of the particles of filter material is then produced during the stage of unblocking with air and water; when the level of the air/water emulsion reaches the spillway of the wash-water collection gutter, a large proportion of the filter mass is then entrained in the gutter.
To avoid this, a known technique consists in raising considerably the level of the spillway of the wash-water collection member so that the entrained material has time to settle before spilling into the gutter. This solution has the immediate disadvantages of considerably increasing the height of the structures as well as the time and water volumes which are necessary.
Various solutions have already been proposed to the very general problem of the backwash water collection member and removal of water which has been used for washing granular filter material on filters of the kind in question, whatever their type, in particular when the filter mass consists of material of low density, without the collection and the removal of this water being accompanied by an untimely entrainment of the material.
One of these solutions (U.S. Pat. No. 4,479,880) consists in equipping the wash-water collection gutters with systems which divert the rising air and water streams. The streams which are thus diverted arrive opposite plates comprising a perforated horizontal part and an inclined part.
Above the perforated plates, the entrained material separates from the air bubbles to which it is attached and settles towards the filter bed in the region bounded by the inclined part of the plate.
This solution has many disadvantages, the chief of which are the following:
The air diverted in the first step rises along the inclined plates and passes through the perforated plate, causing damming of the flow towards the spillway of the gutter and preventing the material from settling towards the filter bed. The so-called settling zone then becomes turbulent and the settling action of the material becomes impossible. On the contrary, the particles of material separated from the air are once again entrained in this turbulence towards the spillway.
Another known solution (U.S. Pat. No. 4,478,726) consists of a device consisting of a horizontal plate placed opposite the spillway of a wash-water collection gutter. In these conditions, the rising water and air are diverted by the plate in line with the spillway. The material detaches itself from the air bubbles at the end of the plate and then settles onto its horizontal part and returns inside the filter by means of a space provided in line with the spillway.
The greatest disadvantages of such a device are the following:
as in the preceding solution, the air which is removed at the end of the plate causes damming of the flow towards the spillway and, above all, a high turbulence in the region where settling of the particles of material should take place, tending to resuspend them;
in addition, the horizontal plate defines a settling zone which must be very large because the settling rates of the filter materials employed are low and the particles are entrained by the overspill flow. For settling to take place, the plate must be very large in size;
the air accumulated under the plate can no longer be removed. This quantity of air, which becomes increasingly larger in the course of the washing, prevents the recirculation of the particles of material towards the filter bed. An accumulation of material is then produced opposite the spillway and the material is then entrained into the spillway and from there to the gutter;
when the air and water washing is stopped a large quantity of material remains on the plate without any possible return towards the filter mass. This material can then be removed only by the additional fitting of complicated or costly flushing or sweeping devices.
Yet another solution (U.S. Pat. No. 4,076,625) consists of a device fitted to the wash-water collection gutters, in the form of symmetrical baffles serving as deflectors of the water and air flow.
While, by virtue of this device, no accumulation of granular material takes place, this device nevertheless presents a major disadvantage during the flushing stage. In fact, the flushing water intended to remove out of the filter the result of the unblocking with air and water is itself also diverted by the baffles. The flow can take place only between the baffles, to spill over into the water collection gutter. The quantity of flushing water accumulated during the unblocking between the devices fitted to each gutter represents a large volume of water which cannot be flushed or which at least will be flushed at the cost of a very long flushing period. In addition, the flow in the space provided between the baffles acting as deflectors is countercurrent to the fluid and the material. The sizing of these deflecting members results in bulky devices which require very great heights of water above the material.
Finally, such a system cannot be fitted to filters with a single lengthwise spillway.
A possible known recent solution (French Patent 85/05,986) relates to a device employing a plurality of vertical plates, or dishes, at uniform intervals, which are arranged in a quincunx pattern and are situated upstream of a settling zone.
The particles of material separated from the air bubbles as they pass between the dishes settle in a "tranquil"zone and return towards the filter bed by means of a calibrated space provided under the dishes.
The washing water then spills over into a collection gutter after the settling zone.
This solution is applicable to materials with a density of the order of 1,700 kg/m.sup.3. However, in the case of materials with a density of the order of, or lower than, 1,500 kg/m.sup.3 and whose settling rate is of the order of 50 m/h, the settling zone to be provided would have to be very large in size, thus adding a penalty to the cost of the structure.